French Patent No. 763,681 to Pierre Arthur Picard discloses an article theft detection apparatus of the type to which this invention applies. As described in that patent, articles to be protected from theft are provided with responder targets in the form of thin strips of material having a high magnetic permeability and low coercivity and which are rapidly and repeatedly driven into and out of magnetic saturation in the presence of an alternating magnetic interrogation field. The strength of the magnetic interrogation field exceeds the coercivity of the responder target, so that the magnetization of the responder target is flipped when the field alternates. An interrogation antenna is provided at an interrogation zone in a passageway leading to the exit of a store or a protected area in a store; and means are provided to cause the interrogation antenna to generate an alternating magnetic field at a given frequency and at an intensity sufficient to saturate a responder target in the interrogation zone. As a result, the responder target itself produces alternating magnetic fields. A receiver antenna is also provided at the interrogation zone to receive the magnetic fields produced by the responder target. The receiver antenna is connected to a receiver which is tuned to detect signals produced by the responder target; and an alarm is connected to the receiver to be activated when such detection takes place.
U.S. Pat. No. 4,222,517 issued Sep. 16, 1980, to Richardson discloses a deactivatable responder target comprising a signal element in the form of a ribbon of relatively low coercivity ferro-magnetic material and having coupled thereto control elements of a ferro-magnetic material which may be permanently magnetized by a control signal to saturate the strip of first ferro-magnetic material, thereby preventing the first ferro-magnetic material from responding when interrogated by a periodic magnetic signal. The coercivity of the control elements are at least as high as the peak amplitude of the interrogation field. Typically, the coercivity of such control elements is 50 Oe to 150 Oe, a factor 10 to 30 higher than a typical interrogation field of 5 Oe. The control elements are typically formed from the alloy of cobalt, vanadum and iron known as vicalloy.
To assemble the responder targets, long continuous ribbons of the constituent elements are formed, the elements are bonded to one another between compression rollers, and the responder targets are cut from the bonded strip.
The use of materials that could be coated in strips rather than chopped would improve the manufacturability of the responder targets. U.S. Pat. No. 3,765,007 issued Oct. 9, 1973 to Elder describes the use of a thin coating of .gamma.-ferric oxide powder in a vinyl chloride binder as a control element. Experiments conducted by the present inventors have shown, however, that .gamma.-ferric oxide powder has too high a coercivity to be useful in a responder target of the type shown in U.S. Pat. No. 4,222,517.